The liquid crystal display element, which is a display element featuring low profile, lightweight and low power consumption, has been widely used as a display screen of word processors and of TVs hitherto. Of the known liquid crystal display elements, the polymer dispersion type liquid crystal display element using light-scattering mode, which requires no polarizers and also requires no alignment layer treatment to substrates, enables a simplified structure and bright and good contrast ratio display. In particular, when the polymer dispersion type liquid crystal display element is applied to projection type liquid crystal display adapted to project images on the screen, a large image of bright and excellent contrast ratio can be easily created on the screen, and accordingly the use of the polymer dispersion type liquid crystal display element in this field is being progressing.
The polymer dispersion type display element has however a delay in development, as compared with the liquid crystal display elements of TN (Twisted Nematic) mode and STN (Super Twisted Nematic) mode and still has the following disadvantages. Since the polymer dispersion type liquid crystal is such that microscopic liquid crystal droplets of micron order are confined in the polymer matrix, liquid crystal molecules in the liquid crystal droplets are affected by physical restrictive force (hereinafter it is called as "anchoring") from an interfacial boundary of the polymer matrix. Because of this, the polymer dispersion type liquid crystal display element is poorer in response of the liquid crystal molecules to electric field than other types of liquid crystal display elements and has a hysteresis that creates a difference in transmittance of the element between at a raised voltage and at a dropped voltage. Further, since the anchoring strength varies depending on temperature of the element, as ambient temperature around the element varies, the transmittance characteristics of the element relevant to the response to electric field and to driving voltage vary considerably. Due to this, although the polymer dispersion type liquid crystal display element holds promise as the coming generation liquid crystal display element, the element of high reliability with satisfactory performance have not yet been realized in the present circumstances.
Following techniques for the polymer dispersion type liquid crystal display element have been hitherto disclosed.
(1) Disclosed by Flat Panel Display '91, on page 221, published by NIKKEI BP and others is the technique according to which after a compatible mixture of a liquid crystal material and polymerizable monomer is injected in between two opposing substrates, the compatible mixture is irradiated with ultraviolet from above of the substrates under a given temperature condition, to polymerize the monomer while phase separation of the liquid crystal is produced, to thereby produce the polymer dispersion type liquid crystal in which liquid crystals are dispersed in polymer matrix or are dispersed with continuously linked to each other.
(2) Disclosed by Japanese Laid-open Patent Publication No. Hei 5(1993)-158020 is the technique of controlling phase separation by concentration of polymerization initiator in the liquid-crystal-polymer mixture, polymerization temperature and intensity of ultraviolet being all controlled simultaneously.
(3) Disclosed by Japanese Laid-open Patent Publication No. Hei 5(1993)-224180 is the technique of controlling a rate of polymerization of monomers in the guest host type of polymer dispersion type liquid crystal display element.
(4) Disclosed by Japanese Laid-open Patent Publication No. Hei 5(1993)-158020 is the technique of improving intensity of ultraviolet from a conventional range of about 10 mW/cm.sup.2 (cf. Symposium on page 414 of The 21.sup.st Liquid Crystal Symposium by Mr. Fujikake and others, for example) to the range of from 0.5 mW/cm.sup.2 or more to 100 mW/cm.sup.2 or less.
(5) Disclosed by Japanese Laid-open Patent Publication No. Hei 5(1993)-127174 is the technique according to which intensity of ultraviolet is set to be 15 mW/cm.sup.2 or more when a radical polymerization initiator is used, while on the other hand, the intensity of ultraviolet is set to be in the range of from 100 mW/cm.sup.2 or more to 150 mW/cm.sup.2 or less when an ionic polymerization initiator is used.
(6) Disclosed by Japanese Laid-open Patent Publication No. Hei 6(1994)-194629 is the technique on a surface temperature of a liquid crystal panel irradiated with ultraviolet, according to which polymerization is produced under temperatures higher than thermal phase separation temperature by a minimum requiring extent, to allow for solubility limit of liquid crystals.
However, these conventional techniques were not enough to solve the abovesaid problems satisfactorily and were also disadvantageous in that it takes much time to accomplish the phase separation by, for example, irradiation of ultraviolet (it takes much time to solidify polymer matrix), due to which great variations in size of liquid crystal droplets and interval between neighboring liquid crystal droplets are caused. Also, the conventional techniques involve the problem that since the anchoring strength of interface liquid crystal/polymer is not adequately adjusted, the response to electric field is not sufficient and the optical hysteresis in high temperature range is as large as 3 to 5% and also the optical hysteresis in low temperature range (less than 10.degree. C.) increases further.
At present, what is physical value that controls the optical hysteresis directly is not thoroughly clarified. For this reason, the measurements to improve the optical hysteresis effectively have not yet been found out.